1. Field of the Invention
The present invention relates to an optical multiplexing technology for multiplexing a plurality of optical signals of different wavelengths. More particularly, the invention relates to an optical multiplexing apparatus for reliably multiplexing a plurality of optical signals having narrow wavelength intervals with a simple constitution, and to an optical multiplexing method.
2. Description of the Related Art
In the optical communications In recent years, it has been desired to increase the transmission capacity, and a wavelength division multiplexing (WDM) system has been developed as one of the means. In the optical transmission system according to the WDM system, in general, optical signals of plural wavelengths are transmitted through a single optical transmission line, and are demultiplexed or multiplexed in accordance with their wavelengths while being amplified through an optical amplifier so as to be transmitted to a desired terminal station.
In the optical transmission system according to the WDM system, efforts have been made to further increase the transmission capacity by decreasing the wavelength intervals (channel intervals) of a plurality of optical signals. For example, there has been proposed a system in which the wavelength interval is decreased to be from 100 GHz to 50 GHz. In the above optical transmission system, technology becomes necessary for widening a transmission band without the occurrence of crosstalk among the channels, so that the reliable multiplexing or demultiplexing of the optical signals can be performed in accordance with the wavelengths.
As the conventional optical multiplexing technology for multiplexing optical signals of narrow wavelength intervals, there can be exemplified the one disclosed in Japanese Unexamined Patent Publication No. 58-70652 (Japanese Patent No. 1427285). This publication is constituted by the combination of an optical multiplexing circuit having wavelength dependency and an optical multiplexing circuit without wavelength dependency. For example, optical signals of every other wavelengths are multiplexed by a first optical multiplexing circuit having wavelength dependency, optical signals of the remaining wavelengths are multiplexed by a second optical multiplexing circuit having wavelength dependency, and output light from the first and second optical multiplexing circuits are multiplexed together by an optical multiplexing circuit without wavelength dependency.
However, when the wavelength intervals of the optical signals to be multiplexed are set to be as very narrow as, for example, 50 GHz, the influence of nonlinear optical effect becomes a problem together with the occurrence of crosstalk among the channels. Concretely speaking, when the optical signals of neighboring wavelengths are under the same polarization state in a wavelength interval of 50 GHz, it is considered that the power levels of the respective optical signals are susceptible to the influence of the nonlinear optical effect such as the four light wave mixing. In order to relax the nonlinear optical effect, it is effective to differ the polarizing state of the neighboring wavelengths and, particularly effective to employ a cross polarization system.
However, it is not easy to multiplex the optical signals having narrow wavelength intervals while maintaining their polarization states. When an existing optical device is considered, such as an arrayed waveguide grating (hereinafter referred to as AWG), it is relatively easy to multiplex a plurality of optical signals in the identical polarization state while maintaining their polarization state, but it is very difficult to realize the multiplexing when they are different in polarization states. Besides, even if an optical device is realized which is capable of multiplexing such optical signals, its cost will be very high.
Further, even if the nonlinear optical effect is relaxed by the employment of the cross polarization system, crosstalk occurs among the channels accompanying a decrease in the wavelength interval unless the optical device multiplexing the optical signals exhibits sharp filter characteristics, to deteriorate the transmission characteristics. The sharp filter characteristics referred to here stand for that the transmission band width of the filter is narrow to a sufficient degree.
According to the conventional optical multiplexing technology disclosed in the above Japanese Unexamined Patent Publication No. 58-70652, optical signals of neighboring wavelengths are multiplexed while maintaining the cross polarization state. It is possible to decrease the influence due to the nonlinear optical effect accompanying a decrease in the wavelength interval but it is difficult to sufficiently suppress the occurrence of crosstalk among the channels. That is, the above conventional technique uses interference-film type optical devices as the first and second optical multiplexing circuits. In general, however, since the interference-film type optical devices have poor filter characteristics, the wavelength interval that can be coped with is restricted. When the wavelength interval becomes as narrow as about 50 GHz, although the optical devices having a wavelength interval of twice as wide can be used as the first and second optical multiplexing circuits, it is impossible to avoid deterioration in the transmission characteristics due to crosstalk only by simply multiplexing a set of cross polarized optical signals without any contrivance for suppressing the crosstalk in the third optical multiplexing circuit.